Lead screw shaft with adjustable pitch

ABSTRACT

A lead screw apparatus is disclosed that permits fine adjustments to be made in the screw pitch. A lead screw shaft having N threads, an initial length L, and a spring constant K1 is coaxially connecting to a spring having a spring constant K2 substantially smaller than K1. By deforming the spring by a distance delta 2, a force equal to delta 2K2 is generated. This force is transmitted to the lead screw, producing a change in length of the screw of delta 1 where delta 1 is equal to delta 2K2/K1. Since K2 is substantially smaller than K1, delta 1 will be substantially smaller than delta 2 by the same order of magnitude. Thus a large change delta 2 in the length of the spring will change the pitch of the lead screw from N/L to N/L + OR - + OR - delta 1.

United States Patent Rongved May 2, 1972 [54] LEAD SCREW SHAFT WITH ADJUSTABLE PITCH [72] lnventor: Leil Rongved, Summit, NJ.

[73] Assignee: Bell Telephone Laboratories Incorporated,

Murray Hill, NJ.

[22] Filed: Dec. 29, 1970 [21] Appl. No.: 102,421

[52] US. Cl. ......74/424.8 R, 74/400, 74/441 [51] Int. Cl. ..F16hl/18, F1611 35/08, Fl6h 55/18 58] Field of Search ..74/424.8 R, 400, 441

[56] References Cited UNITED STATES PATENTS 2,690,682 10/1954 Passman ..74/44l 3,559,500 2/1971 Galonska et al ..74/424.8 R

Primary Examiner-Leonard H. Gerin Attomey-R. J. Guenther and Edwin B. Cave [57] ABSTRACT A lead screw apparatus is disclosed that permits fine adjustments to be made in the screw pitch. A lead screw shaft having N threads, an initial length L, and a spring constant K is coaxially connecting to a spring having a spring constant K, substantially smaller than K By deforming the spring by a distance 6 a force equal to 8 K, is generated. This force is transmitted to the lead screw, producing a change in length of the screw of 6, where 8, is equal to 6,K /K,. Since K is substantially smaller than K 8, will be substantially smaller than 8 by the same order of magnitude. Thus a large change 8 in the length of the spring will change the pitch of the lead screw from N/L to NHL-6,.

12 Claims, 4 Drawing Figures I M ii WW /7 4" PATENTEUHAY 2 I972 3, 659.473

FIG. 3

W /WWM 36 /Nl/E/V70R L. RONGVED ATTOPNFV LEAD SCREW SHAFT WITH ADJUSTABLE PITCH BACKGROUND OF THE INVENTION The change of rotary motion into rectilinear or translational motion has long been accomplished by means of a lead screw and mating nut. Depending on the pitch of the screw, a specific number of revolutions of the screw will produce a known rectilinear movement of the nut. Examples of the application of such an arrangement are the movement of cutting tools along the workpiece of an automatic lathe and the socalled general purpose indexing table used by machinists to obtain x-y positioning of a workpiece.

In such applications, extreme accuracy in the fabrication of the lead screw is not necessary since the relationship between rotation of the lead screw and rectilinear movement of the engaging nut or table is not critical. However, in a field such as integrated circuit manufacture, precise positioning of a workpiece is essential. Whether we deal with the generation of a mask or printing pattern used to fabricate integrated circuits or with the process of welding the beam-lead terminals of such circuits to a supporting structure or other circuit, the precise positioning of the equipment involved is extremely critical.

In such applications where the rotation of the lead screw is expressed as revolutions and the movement of the mating nut or table is expressed as x microns, the tolerances permitted are such that for a range of 0 from 0 to 80 revolutions, x may not vary from the theoretical solution of the equation x 3,500 0 by more than l.5 microns. Obviously, manufacturing precision required for the manufacture of such lead screws, even if successful, would be prohibitively expensive. Further, a change of only a few degrees between the temperature under which the lead screw was fabricated and the the temperature under which it is used would completely alter the pitch accuracy.

ln fabricating such screws, two distinct sources of error occur. First, there will be some departure from linearity of the x, 0 curve for a given lead screw. Secondly, from one screw to another there will be some departure in the slope of the x, 0 curve. Using present commercially practical tolerances with a minimal amount of selection between several screws, the precise tolerance indicated above could be met if only the first source of error were present. When combined with the second source of error however, it becomes impractical or impossible to attain the desired tolerances despite the selection of screws and fabrication to the best commercially available tolerances.

. Fortunately, I have developed a lead screw mechanism having its x, 0 slope readily adjustable. As a result, using commercial tolerances presently available, it is possible to adjust the pitch of an already fabricated lead screw to obtain the tolerance indicated above as being a maximum allowable deviation.

The x, 0 slope is adjusted by changing the overall length of the lead screw. Since the number of threads on the screw will not change, the pitch of the screw changes as the length changes. The adjustment to the length of the screw must be precise, so directly compressing the screw would be ineffective because the compressing mechanism would then require precise adjustment.

Even assuming that such a precise adjustment were possible, once set, the mechanism s adjustment would be subject to variation due to the presence of dirt on the threads of the mechanism, vibration, movement and shock, and many other minor variables. My mechanism provides a large mechanical advantage, on the order of several magnitudes. As a result, a large change in the compressing mechanism is required to produce the precision change in the length of the lead screw (for example, the ratio between the two changes may be Since the change in the compressing mechanism is large, any slight variation occurring in the adjustment of the compressing mechanism will have a negligible affect on the length of the lead screw. The slight variation would produce a corresponding inaccuracy in the overall length of the lead screw equal to only 10" times the slight variation in the compressing mechanism. Even with extremely small adjustments and precision movements, an error of 1 part in 10 or 100,000 is negligible when the overall length of the lead screw is considered.

SUMMARY OF THE INVENTION The lead screw is connected to a spring and to a member for deforming the spring and applying the force generated thereby to the screw. Since the spring constants of the spring and the lead screw differ by orders of magnitude, the change in length of the lead screw will be corresponding orders of magnitude less than the change in length of the spring. Thus a large change in spring length will produce a slight change in the length of the lead screw with a resultant precision change in the pitch of the screw. As a result, the adjustment necessary to precisely change the pitch of the screw does not require precise movement of the adjusting mechanism.

BRIEF DESCRIPTION OF THE. DRAWING FIG. I shows a cross-section of an embodiment of my invention in which the force applied to the lead screw is obtained by compressing the spring;

FIG. 2 shows a schematic representation of the embodiment shown in FIG. 1',

FIG. 3 shows a cross-section of an embodiment of my invention in which the force applied to the lead screw is obtained by extending the spring; and

FIG. 4 shows a schematic representation of the embodiment shown in FIG. 3.

DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENTS Referring now to FIG. I, a lead screw 10 is shown with a mating nut or table slide ll engaging the threads. Rotation of screw 10 will cause nut 11 to move laterally some distance, depending on the pitch of the screw and the amount of rotation. To change the distance laterally that nut 11 would move in response to a particular amount of rotation, it is necessary to alter or adjust the pitch of lead screw 10. Since the number of threads N of screw 10 is fixed, to change its pitch the overall length must be adjusted.

The mechanical apparatus shown in FIG. 1 is depicted schematically in FIG. 2. To adjust the length of screw 10 by a small distance 8,, it must be subjected to a force equal to 8, times the spring constant K, of the screw. This force is generated by compressing a softer spring 15 having a spring constant K by a distance 0 Adjusting nut 17 is threaded down on the threaded right end of rod 16. The left end of rod 16 is threaded into a fixed base 12. Since nut 17 bears on one end of spring 15, compressing the spring by a distance 6 will generate a force equal to 8 1( Since the other end of spring 15 bears on screw 10, and since screw 10 bears against fixed base 12 at its other end, the generated force 6 K, is also equal to 8,K,. Since spring 15 and screw 10 act in series, the total applied force F generated by nut 17 must equal (K S,)[S,/( K,+K to produce deformations S and S, respectively.

Thus it can be seen that a moderate compression 8 of a soft spring 15 having a spring constant K will cause a small compression 8, of screw 10 which has a spring constant of K,. The small compression 8, of screw 10 due to the application of force F A by adjusting nut 17 will cause its overall length to decrease from L to L6,. As a result, the pitch of screw 10 will change from N/L to N/(L-8,

FIG. 3 shows an alternative mechanical arrangement which is depicted schematically in FIG. 4. In this arrangement, the spring and screw act as springs connected in parallel. By contrast, the apparatus of FIGS. 1 and 2 acted as two springs connected serially.

In the arrangement of FIGS. 3 and 4, the adjusting force F B is generated by threading adjusting nut 39 down on rod 38. Rod 38 connects to a swiveled clamp 36 which holds the right end of spring 35. The left end of spring 35 is held by fixed base 32. The force F B generated by extending spring 35 a distance 8 is equal to 8, times K the spring constant of spring 35.

Generated force F,, is applied through nut 39 and thrust bearing 37 to the right end of lead screw 30, which has N threads and engages a nut or table slide 31. The left end of screw 30 bears against fixed base 32. Since force F 8 acts to compress screw 30 whose spring constant is K the initial length L of the screw will decrease by a small distance 8,. As a result the length of screw 30 will decrease from L to L6 and the screw pitch will change from N/L to NHL-8 Of course, both arrangements also permit the initial length L of screws 10 or 30 to be increased by an amount 8,. In FIG. 1, an initial tension applied to rod 16 of nut 17 could be decreased by threading nut 17 back off the rod. The length of spring would then increase by a distance 8 resulting in a force relaxation of 8 K, The length of screw 10 would increase by a distance 6 and its pitch will change from N/L to N/( -O- Similarly, the adjusting nut 39 could be backed off rod 38 to relax the initial force applied to thrust bearing 37 by an amount of 8 K This would cause screw 30 to elongate by a distance 8 and its pitch to change from N/L to N/(L+8 It should be apparent now to those skilled in the art that other arrangements of my invention are feasible without departing from the spirit and scope of my invention. Depending on the relative change 6 desired in screw 10 or 30, the spring constant K of spring 15 or 35, respectively, may be selected so that distance 8 is practical and minor variations in precision in the adjustment of distance 8 will have a negligible effect upon the desired adjustment of pitch of the screw.

What is claimed is:

l. A threaded member having changeable pitch comprising a first member having N threads and a spring constant K,;

a second member having a spring constant K and located coaxial to the first member;

means for changing the length of the second member by a distance 8 and means for applying to the first member the force generated by deforming the second member from its original length, so that the initial length L of the first member is selectively changed by a distance 8 equal to (K2/K|)82;

whereby the thread pitch of the first member is changed selectively from N/L to N/(Lt8,).

2. Apparatus in accordance with claim 1 wherein the second member comprises a coil spring and the changing means comprises a threaded rod and engaging nut.

3. Apparatus in accordance with claim 1 wherein the ratio K /K is several orders of magnitude, so that the resulting ratio 8 /8 is the same orders of magnitude.

4. Apparatus for precisely adjusting the pitch of a lead screw comprising a lead screw with N threads and having a spring constant K a spring having a spring constant K means communicating with one end of the lead screw and with one end of the spring for producing an initial stress on the lead screw with a resulting length L; and

means for displacing the other end of the spring by a distance 8 and transmitting the change in stress to the lead screw, thereby causing the length of the lead screw to change by a distance 8 which is equal to (IQ/[( )8 and the pitch of the lead screw to change from N/L to N/(Lt 6 5. Apparatus in accordance with claim 4 wherein the spring is a coil spring located coaxial with the lead screw.

a lead screw with N threads and having a spring constant of 1; a spring having a spring constant K and located so that one end of the spring communicates with one end of the lead screw; and means communicating with the other end of the spring and with the other end of the lead screw for both producing an initial stress in the lead screw resulting in an overall length L and for deforming the spring by a distance 8 thereby altering the stress in the lead screw causing it to change length by a distance 8, equal to [I(J(l(,-l-l(,)]8 and to change pitch from N/L to MIA-8 8. Apparatus in accordance with claim 7 wherein the spring is a coil spring located coaxial with the lead screw.

9. Apparatus in accordance with claim 7 wherein the ratio K lK is several orders of magnitude so that the resulting ratio 8J8, is substantially the same orders of magnitude.

10. A lead screw mechanism having precisely adjustable pitch and comprising a hollow cylindrical member having a length L, a spring constant K,, and N threads formed on the outer periphery thereof;

a coil spring coaxial with the cylindrical member and having a spring constant K difiering from K, by at least several orders of magnitude; and

stress producing means communicating with both the cylindrical member and the coil spring for producing a concurrent strain 8 in the cylindrical member 8 in the spring;

so that the strain 8, produced in the cylindrical member and causing its pitch to change precisely from M1. to N/(L: 8;) will result from the concurrently produced strain 6 several orders of magnitude greater than 8, and related to 8 substantially by the ratio K lK 11. Apparatus in accordance with claim 10 wherein one end of the coil spring bears against one end of the lead screw; and wherein the stress producing means includes a base bearing against the other end of the lead screw;

a rod affixed to the base and extending through the cylindrical member and the coil spring;

and means bearing against the other end of the coil spring and moveably engaging the rod;

so that the compressive strain in the coil spring is directly related to the compressive strain in the lead screw.

12. Apparatus in accordance with claim 10 wherein the coil spring is located within the hollow of the cylindrical member; and wherein the stress producing means includes a base bearing against one end of the lead screw and securing one end of the spring; a threaded rod securing the other end of the spring; and an adjustable member engaging the threaded rod and bearing against the other end of the lead screw;

so that the tensile strain in the coil spring is directly related to the compressive strain in the lead screw.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent: No. 3, 659 H73 Dated May 2 1972 Inventor-(s) Leif ROngved It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

last line, change 'N/L to N/Li+ -6 L 6 Column 2, line 47, change "6 2 Column'fi, line 16, change "N/(L+ to -N/(L+6 Signed and sealed this 28th day of November- 1972.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. ROBERT GOTTSCHALK Attesting Officer Commissionerof Patents FORM PC4050 0&9,

UBCOMM'DC 60376969 IL. GDVIIIIIII 'IIIVIIG OI'ICI 1 "I! 0-3.0410 

1. A threaded member having changeable pitch comprising a first member having N threads and a spring constant K1; a second member having a spring constant K2 and located coaxial to the first member; means for changing the length of the second member by a distance delta 2; and means for applying to the first member the force generated by deforming the second member from its original length, so that the initial length L of the first member is selectively changed by a distance delta 1 equal to (K2/K1) delta 2; whereby the thread pitch of the first member is changed selectively from N/L to N/(L + OR - delta 1).
 2. Apparatus in accordance with claim 1 wherein the second member comprises a coil spring and the changing means comprises a threaded rod and engaging nut.
 3. Apparatus in accordance with claim 1 wherein the ratio K1/K2 is several orders of magnitude, so that the resulting ratio delta 1/ delta 2 is the same orders of magnitude.
 4. Apparatus for precisely adjusting the pitch of a lead screw comprising a lead screw with N threads and having a spring constant K1; a spring having a spring constant K2; means communicating with one end of the lead screw and with one end of the spring for producing an initial stress on the lead screw with a resulting length L; and means for displacing the other end of the spring by a distance delta 2 and transmitting the change in stress to the lead screw, thereby causing the length of the lead screw to change by a distance delta 1 which is equal to (K2/K1) delta 2 and the pitch of the lead screw to change from N/L to N/(L + or -delta 1).
 5. Apparatus in accordance with claim 4 wherein the spring is a coil spring located coaxial with the lead screw.
 6. Apparatus in accordance with claim 4 wherein the ratio K1/K2 is several orders of magnitude and the related ratio delta 1/ delta 2 is the same several orders of magnitude.
 7. Apparatus for precisely adjusting the pitch of a lead screw comprising a lead screw with N threads and having a spring constant of K1; a spring having a spring constant K2 and located so that one end of the spring communicates with one end of the lead screw; and means communicating with the other end of the spring and with the other end of the lead screw for both producing an initial stress in the lead screw resulting in an overall length L and for deforming the spring by a distance delta 2 thereby altering the stress in the lead screw causing it to change length by a distance delta 1 equal to (K2/(K2+K1)) delta 2 and to change pitch from N/L to N/L + or - delta
 1. 8. Apparatus in accordance with claim 7 wherein the spring is a coil spring located coaxial with the lead screw.
 9. Apparatus in accordance with claim 7 wherein the ratio K1/K2 is several orders of magnitude so that the resulting ratio delta 1/ delta 2 is substantially the same orders of magnitude.
 10. A lead screw mechanism having precisely adjustable pitch and comprising a hollow cylindrical member having a length L, a spring constant K1, and N threads formed on the outer periphery thereof; a coil spring coaxial with the cylindrical member and having a spring constant K2 differing from K1 by at least several orders of magnitude; and stress producing means communicating with both the cylindrical member and the coil spring for producing a concurrent strain delta 1 in the cylindrical member delta 2 in the spring; so thAt the strain delta 1 produced in the cylindrical member and causing its pitch to change precisely from N/L to N/(L + or - delta 1) will result from the concurrently produced strain delta 2 several orders of magnitude greater than delta 1 and related to delta 2 substantially by the ratio K1/K2.
 11. Apparatus in accordance with claim 10 wherein one end of the coil spring bears against one end of the lead screw; and wherein the stress producing means includes a base bearing against the other end of the lead screw; a rod affixed to the base and extending through the cylindrical member and the coil spring; and means bearing against the other end of the coil spring and moveably engaging the rod; so that the compressive strain in the coil spring is directly related to the compressive strain in the lead screw.
 12. Apparatus in accordance with claim 10 wherein the coil spring is located within the hollow of the cylindrical member; and wherein the stress producing means includes a base bearing against one end of the lead screw and securing one end of the spring; a threaded rod securing the other end of the spring; and an adjustable member engaging the threaded rod and bearing against the other end of the lead screw; so that the tensile strain in the coil spring is directly related to the compressive strain in the lead screw. 